Antimatter.
Antimatter - matter, built of antiparticles. The existence of antiparticles was first predicted in 1930 the British physicist Paul Dirac. Of the Dirac equation for a relativistic electron followed a second solution to its twin, which has the same mass and a positive electric charge.

At the time, was aware of only one positively charged particles - protons, differ greatly in their properties from the electron.

Theorists have come up with clever explanations for these differences, but it soon became clear that the proton has nothing to do with a particle predicted by Dirac.

In 1932, the positively charged anti-electron (now called positrons) discovered in cosmic rays is an American physicist, K. Anderson. This discovery was a brilliant confirmation of Dirac's theory.
In 1955, the new accelerator in Berkeley (USA) E. Segrs, O. Chamberlain and others, discovered antiprotons born in collisions of accelerated protons with the nuclei of copper target.

Prior to this proton with a negative charge had unsuccessfully sought by cosmic rays. In 1956 he was discovered and antineutron. Now we know already a lot of particles, and almost all of them correspond to the antiparticle.

Particles and antiparticles have the same mass, lifetime, spin, but differ in signs of all charges: electric, baryon, lepton, etc. This follows from the general principles of quantum field theory and confirmed by experimental data. From a modern point of view of elementary particles are divided into d in e group. The first of these - particles with half-integer spin: the charged leptons, and their corresponding neutrinos and quarks. All these particles and antiparticles have. Another group - this is the quanta of fields with integer spin, transferring interactions: the photon, intermediate bosons of weak interactions, and the gluons of strong interactions. Some of them are truly neutral, i.e., all the quantum numbers are zero and they are identical to their antiparticles, while others also form a pair of particle - antiparticle. It is easy to see now that all the baryons composed of three quarks must have an antiparticle, for example, has the composition of the neutron (udd), an antineutron - (udd). Mesons consist of quark and anti-quark, and, generally speaking, also have antiparticles. At the same time, there are mesons are symmetric with respect to change in the quark anti quarks, mesons are to be truly neutral.